The invention relates to a primary shaping method for a component having at least one microstructured functional element, which is configured intentionally with a defined reliefed structure at a defined point on the surface of the component in order to specifically fulfill a function and which comprises, in at least one spatial direction, a characteristic dimension in the micrometer range, said component being shaped from a substantially metallic material using a molding tool. As compared to a component having a purely macroscopic function, such a component additionally comprises a reliefed, microstructured and, as a result thereof, functionalized surface.
Shaping methods substantially form the three-dimensional shape of a component without changing its mass. Primary shaping methods, which, from the liquid or ductile or from the granular or powdery aggregate state (in the classification in accordance with DIN 8580)—create the first three-dimensional shape, and primary shaping methods, which change a three-dimensional shape in the solid aggregate state through pressure, tensile and compressive loads, bending or pushing loads (in the classification in accordance with DIN 8580) are known.
In this context, the molding tool is the tool by means of which the three-dimensional shape of the component is imposed. In the context of the shaping method, the material deposits in its respective aggregate state on the surface of the molding tool—for example of a mold cavity in a casting method. A negative of a functional element formed on the surface of the molding tool is thus directly formed in the functional element on the surface of the component that fits the mold. From the great number of known primary shaping methods forming the three-dimensional shape of components from a substantially metallic material, using a molding tool, casting, sintering and liquid-phase sintering will be mentioned in particular herein.
A microstructure is a reliefed surface structure comprising, in at least one spatial direction, a characteristic dimension in the micrometer range—meaning substantially of considerably less than 1 mm. Such a characteristic dimension is for example the depth of an edge offset downward with respect to a surface or the width of a rib placed onto a surface.
Microstructures have been found to be advantageous in many respects. Microstructured surfaces are utilized for example in tribological applications, from aero or fluid-dynamic point of views, because of specific visual properties, for controlling the wettability or non-wettability with liquids and for promoting or preventing organic growth.
A functional element is an element that is intended to perform a defined function thanks to a defined shape. More specifically, a functional element is not an element that performs the defined function at a fortuitous location on a component or by having a fortuitous shape.
A microstructured functional element accordingly is an element that is intentionally and selectively configured to have a defined structure at a defined point on the surface of a component and that has a dimension in the micrometer range which is characteristic of the function.
A periodic or almost periodic arrangement of microstructured functional elements is considered to be a microstructured surface texture, a defined detail of a surface having microstructured elements as the functional region or a functional element (itself composed of smaller functional elements).
Thanks to its reliefed structure in the region comprising the functional element, the surface of the component is either functionalized or has its function optimized there. The flow guidance at the surface of a turbine bucket may for example be significantly improved by a microstructured surface texture.
Beside the material and its microstructure, as well as the macroscopic shape, it is the surface that determines the properties of a component. On the one side, perfectly smooth surfaces have come to represent technical perfection, on the other side, minute structures provide a surface with dirt- and water-repelling functions through what is known as the “Lotus effect”, spectacle lenses can be provided with an additionally antireflection coating applied to the surface thereof. The functions of light reflection, flow resistance, heat transfer and friction of a component's surface may also be selectively influenced by microscopic surface structures.
The manufacturing of large microstructured surfaces on plastic materials—at least on planar surfaces—is to be considered to be largely known: surface structures in the micrometer range are formed and replicated, using the comparatively simple methods of soft lithography under normal atmosphere. The PDMS (polydimethyl siloxane) stamps used in soft lithography hereby form structures with characteristic dimensions of less than 100 nm (H Schmid, B Michel, Macromolecules 33, 2000, p. 3042). In the field of plastic materials, optical data carriers constitute moreover an impressive example of a product with a microstructured surface: CD-ROM disks, manufactured on a large scale by injection molding, have structures of less than 1 μm, DVD having structures of even less than 500 nm. The production of a plurality of other surface structures, including but not limited to, biomimetic structures such as “shark skin” on polymers is already known.
On ceramic materials, surface structures in the micrometer range are also reproduced neatly, as has been exemplified by a kind of slip casting in structured PDMS stamps (U P Schönholzer et al. “Micropatterned Ceramics by Casting into Polymer Moulds” J. Amer. Soc. 85 7, 2002, p. 1885). Also known is the manufacturing of structures having a size of as little as 10 nm by pressing into the molten surface of a silicon wafer a quartz disk patterned using electron beam lithography (S Y Chou, Ch Keimel, Jian Gu “Ultrafast and Direct Imprint of Nanostructures in Silicon”, Nature 417, 2002, p. 835).
Vapor deposited coatings on metallic components having a roughness in the nanometer range are also known; these coatings however do not have a geometrically defined structure. On the other side, the function of a metallic surface can be influenced within narrow limits by selective, geometrically defined patterning on the microscopic scale using chemical etching, micromachining or laser patterning. Structures geometrically defined in the nanometer range can be produced on small surface portions of a metallic component's surface using electron beam lithography.
FDA standardizes in detail indications as to the microstructure of the surface for approving a modified metallic surface of orthopedic implants: the thickness of a coating, the pore diameter, the shape and dimensions of the material between the pores and the volume percent of the voids must be determined in complex test series through their statistical average and limit values as well as through their standard deviation (U.S. Food and Drug Administration: Guidance Document for Testing Orthopedic Implants with Modified Metallic Surfaces Apposing Bone or Bone Cement. February 2000, http://www.fda.gov/cdrh/ode/827.html.
As compared with plastic components with microstructured surfaces, the production of metallic microstructured functional elements on metallic components manufactured using primary shaping methods is interesting from many point of views because they are less prone to wear and exhibit higher hardness and because they may additionally be utilized at higher temperatures. However, the known methods for modifying the surface structure can be utilized for some few special applications only if they are to be economically efficient, this being due on the one side to their complexity and on the other side to the demands placed on testing and documentation because of the statistical distribution of the properties.
The document DE 101 54 756 C1 discloses a primary shaping method, using a molding tool in the surface of which microscopic cavities are formed by anodic oxidation, directly and without a model—meaning so as to be statistically distributed. The document EP 0 838 286 A1 discloses an investment casting method using a wax model on the surface of which molten wax is sprinkled to form a microporous surface structure, which again is statistically distributed. The document DE 38 31 129 A1 discloses a method for manufacturing a casting mold on the basis of a thermally sensitive model such as textiles, plastic material, wood or leather, with the surface structure of the model being reproduced in the casting mold. The methods disclosed in these documents provide for statistically distributed surface structures but not for a defined reliefed microstructured functional element at a defined point on the surface of the component for selectively performing a function.
In the wider context of the invention, there is known from U.S. Pat. No. 6,511,622 B1 to use a wax “filled” with particles for manufacturing a wax model which in turn is used for investment casting in order to minimize the formation of microscopic defects in the surface of the wax model. The document DE 43 07 869 A1 discloses a primary shaping method for manufacturing a microscopic body as it is utilized in precision engineering, micromechanics, microoptics and microelectronics; it does not disclose the formation of a microstructured functional element on a—macroscopic—component, though.